A multi-stage flash evaporator is the main component of a seawater desalination plant for producing distilled water from sea water. Most evaporators for large capacity desalination plants are of the cross tube type with unit capacities in the range of 10-20 mgd (million gallons per day).
Multi-stage flash evaporators are comprised of a number of flash stages, typically between 15 and 30, where each flash stage is mainly comprised of a flash chamber and a condenser tube bundle. While heated saltwater enters the first flash stage at its highest temperature, the saltwater flashes down and releases some vapor which is then condensed on the condenser tube bundle and collected as distillate. As this process is repeated in each flash stage, the temperature of the saltwater is successively reduced toward the last flash stage while the salt concentration is increasing. The coolant used in the condenser tube bundles enters with the lowest temperature into the condenser tube bundle of the last flash stage and travels through the condenser tube bundles to the first stage, whereby its temperature increases in each tube bundle relative to its temperature in the previous tube bundle. The coolant discharged from the condenser tube bundle of the first flash stage (hot end of the evaporator) is heated in a separate heat exchanger, commonly described as the heat input section, by an external heat source to the top temperature. The coolant discharging from the heat input section then enters the flash chamber of the first flash stage at its top temperature.
The multi-stage flash desalination system may be designed as a “once through” process in which the same coolant is used for all condenser tube bundles in the evaporator flash stages. In other words, the coolant travels through all tube bundles arranged in series, starting from the coldest flash stage to the hottest flash stage.
The most common design concept for multi stage flash desalination plants is the “brine re-circulation” system, in which the evaporator consists of a heat recovery section and a heat rejection section. The heat rejection section comprises a number of flash stages on the cold end of the evaporator, typically 2 to 3 stages, in which fresh seawater is commonly used as a coolant for the tube bundles. In the heat recovery section, which occupies the larger number of flash stages in the evaporator, the heat released from the flashing brine is recovered by the coolant in the tube bundles and used to bring the coolant toward the desired top temperature. A mixture of concentrated brine from the last flash stage and fresh seawater from the coolant discharge of the heat rejection section, commonly described as re-circulating brine, is used as coolant for the heat recovery section. The fresh seawater portion replaces the amount of distillate and concentrated brine discharged from the last flash stage. It is treated in order to limit scaling of the heat exchanger surface and to limit corrosion in the system.
Individual types of evaporators may be differentiated by the tube bundle configuration such as long tube type evaporators and cross tube evaporators. In the long tube type evaporator, the condenser tube bundles are oriented in the flow direction of the flashing brine in the flash stages. This type of evaporator is commonly built in a number of individual modules, where each module contains several flash stages. Each module has one tube bundle with a tube sheet and water box on each end. Individual sections of the tube bundle serve as the condenser tube bundle for the individual flash stages in such module. Tube bundles of the individual modules are typically arranged in serial flow communication.
In the cross tube evaporator, the condenser tube bundles are oriented perpendicular to the flow direction of the flashing brine. In most cross tube type evaporators, each flash stage has an individual tube bundle with tube sheets and water boxes. The tube bundles are typically arranged in a serial flow communication.
In the double stage cross tube type evaporator, a pair of flash stages is equipped with one single pass tube bundle. The partition walls between the two flash stages are arranged such, that the tube bundle is divided at about half the tube length, so that the vapor from the flash stage with the lower vapor temperature condenses on the colder half of the tube bundle, while the vapor from the flash stage with the higher vapor temperature condenses on the warmer part of the tube bundle. With this configuration, the vapor released from the brine uniformly over the width of the flash stage has to travel in one flash stage to one half of the tube bundle and in the following flash stage to the other half of the tube bundle, which creates significant transverse vapor movements in the flash stages.